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Abstract:
The Singhbhum Craton, one of five major Archean cratons in the Indian subcontinent, contains abundant well-preserved Paleoarchean supracrustals and granitoids. This study presents zircon U-Pb ages and whole rock
geochemistry of tonalite-trondhjemite-granodiorites (TTGs) and granites from the Bonai Granite Complex (BGC) and Older Metamorphic Tonalite Gneiss (OMTG), which are separated from each other by the Jamda-Koira-Noamundi Iron Ore Group (IOG) supracrustals. Emplacement ages obtained in this study indicate that a major episode of TTG magmatism took place in BGC around 3368 ± 8 Ma (1σ), followed by granitic magmatism around 3331 ± 33 Ma (1σ). In contrast, a TTG from the Deo Nala area representing OMTG yielded crystallization age of 3312 ± 8 Ma (1σ). The emplacement and evolution of the BGC were coeval with granitoid magmatism from the central part of the Singhbhum Craton. Whole rock geochemical data identify both high- and low-HREE TTGs in both the BGC and OMTG to the west and east of the IOG basin, respectively. The trace element systematics of high-HREE Bonai TTG are similar to those of Icelandic dacites, suggestive of their derivation from a garnet free, plagioclase rich amphibolite. The low-HREE TTGs of the BGC and OMTG were derived from an amphibolite source with varying amounts of garnet. The potassic granites of the BGC were sourced from the older TTGs which had undergone partial melting at a shallow depth. The evolution of the BGC and OMTG can be attributed to the
partial melting under a thickened mafic crust. Dome and keel structures and emplacement ages of granitoids from the west and east of the Jamda-Koira-Noamundi IOG basin, support the origin of these Paleoarchean granitoids in a stagnant lid regime. High geothermal gradients induced by heat supplied by mantle upwelling appear to have induced the melting of the thickened crust, to form the TTG. Delamination induced
mafic–ultramafic underplating resulted in melting of early formed TTGs, to form the younger potassic granites of
the BGC at ~ 3.33 Ga.
